Wtf I might be missing something but- sure he's the asshole! At least concluding from the way he wrote it. It's literally his step daughters party. He acts like she should be on her knees thanking him for helping to raise her. Sure she should have respect for him etc but taking care of her is his duty ffs. Nutter or not, that man is her biological father and she has a special bond with him that can't be denied- the fact that he was abusive, if anything, makes this even more important for her. Talk to her and even perhaps tell her you are uncomfortable, in a neutral way, but if they don't support her in this they fail to do what parents should do and that has everything to do with respect. Stop putting your immature feelings over her on this day that's meant for her. Yeah it may be clear that for many reasons I'm really fed up with the step dad and the mom.